The overarching goal of this career development award is to provide a comprehensive training program to prepare the candidate for an independently funded translational research career focused on the host- pathogen interactions of the major bacterial pathogen, Staphylococcus aureus. The candidate, Dr. Isaac Thomsen, is an Assistant Professor of Pediatrics in the Division of Pediatric Infectious Diseases at Vanderbilt with an advanced degree in patient-oriented research (Masters of Science in Clinical Investigation, MSCI). The proposed research directly examines key gaps in the current understanding of the human immune response to S. aureus. The candidate's near-term goal is to acquire additional expertise in adaptive immunology and molecular epidemiology while continuing a productive line of investigation into the role of the humoral immune response to staphylococcal colonization and disease in children. This award will allow the candidate to fulfill this goal with formal, graduate-level coursework, workshops in key immunological techniques, and protection for 5 years of mentored research. The proposed studies will be conducted over the next several years as part of a comprehensive training experience under the direction of the candidate's co-mentors Drs. Kathryn Edwards and Buddy Creech. Dr. Creech is an established investigator in the field of S. aureus research and was the recipient of the IDSA/SHEA Young Investigator Award in MRSA Research in 2007 and the 2012 PIDS Young Investigator Award. He has published extensively in the field of staphylococcal colonization, molecular epidemiology, antimicrobial resistance, and therapeutics. He will serve as the hands-on, translational science mentor for this project. Dr. Edwards is an internationally recognized expert in pediatric infectious diseases and vaccinology. While maintaining a highly productive research portfolio, Dr. Edwards has also dedicated herself to nurturing the careers of young investigators, and has developed a strong track record for mentoring investigators who go on to obtain independent government funding. Dr. Edwards has received numerous accolades, including the 2006 IDSA Mentor of the Year Award, election to the Institute of Medicine in 2008, and selection as the Outstanding Physician of the Year by the Pediatric Infectious Disease Society in 2011. In addition, Vanderbilt University Medical Center provides a rich environment for research and training of young investigators with centralized oversight of all mentored physician scientists through the Office of Clinical & Translational Scientist Development. Dr. Thomsen's long-term goal is to become an independently funded physician-scientist bridging both human and laboratory studies in staphylococcal immunity research, ultimately identifying novel targets for prevention and treatment of S. aureus. The strong mentorship, additional training, and environment detailed in this proposal will position him to ultimately reach this objective. The research proposal extends the candidate's prior published work on the host and pathogen interactions in pediatric staphylococcal colonization and disease. S. aureus harmlessly colonizes the anterior nares of nearly 1/3 of the population, yet leads to severe, invasive disease in some. The factors that govern this relationship are poorly understood, and this remains a barrier to the development of preventive and therapeutic agents. The candidate's recent work in children with invasive staphylococcal disease led to novel findings regarding the neutralizing antibody response to an important, recently-described toxin known as LukAB. The proposed work is focused on the hypothesis that a high-level, functional antibody response against this toxin is protective against the development of invasive or recurrent S. aureus disease. In Aim 1, the candidate will develop prospective cohorts of children with each of the four major phenotypes of S. aureus infection (colonization, skin infections, recurrent skin infections, and invasive disease), and will examine the anti-staphylococcal antibody profile in these cohorts over time, leading to correlations between specific antibody titers and clinical disease characteristics. In Aim 2, well-established functional antibody assays will allow the determination of antibody function (in the prevention of neutrophil lysis by staphylococcal exotoxins) and the correlation of this neutralizing activity with clinical phenotypes. In Aim 3, th candidate will extend his work on the development of human-derived monoclonal antibodies against LukAB. By determining the binding and neutralizing capacity of anti-LukAB antibodies against each subunit of the toxin, new insights will be gained into the mechanism of the cytoprotective effects of naturally occurring anti-LukAB antibodies following S. aureus disease in humans. Key collaborators in this work include Dr. James Crowe, a world-renowned immunologist at Vanderbilt and an expert in human monoclonal antibodies, and Dr. Victor Torres at the New York University School of Medicine, a well-recognized expert in staphylococcal pathogenesis and S. aureus exotoxins. This project will advance an understanding of the antibody response to S. aureus colonization and disease and will examine the potential of specific staphylococcal virulence factors as targets for future vaccines or novel therapeutic agents.